This is definitely not manufacturer approved! The bearing cups on a Shimano XT Hollowtech II bottom bracket have "DO NOT DISASSEMBLE" written on them. Why? I don't know? Probably because they want you to buy a whole new set of cups for 30 quid or so instead of a couple of the standard bearings they hide for a fiver or so (try your local bearing supplier or eBay)?
If you want to ignore Shimano's instructions? Here's how. This is done on an XT setup but others are similar. Obviously I doubt any warranty would be honoured if you do this? You proceed at your own risk!
You'll need two 6805 bearings (dimensions 25x37x7mm) - the more sealed and the better quality - the better.
- 5mm allen key
- Hollowtech II Install Spanner & Tool
- Small flat screwdriver
- Punch or suitable drift
- Headset press (or threaded bar and suitable washers/spacers)
- Supports (couple of short lengths of 3x2 wood are fine) for holding the cups while knocking bearings out
- Teflon grease
- Copper grease
- (rubber mallet)
- (Stanley knife)
2. Remove the drive sidecrank by pulling it out. A tap on the other end of the axle with a rubber mallet may be required?
3. Remove the bottom bracket cups using the Hollowtech II Install Spanner. Note: the non-drive side has a standard thread. The drive side has a left handed thread. The cups usually have markings on them to help with this! Remember what spacers you have on which side.
4. Prise the plastic covers off the cups. Be careful! There is a rubber seal underneath which you don't want to damage!
5. Prise the seals off the bearings. Be careful not to damage or bend them.
6. Support the cups and knock out the bearings from the back. Work your way gradually around the bearing slowly until it pops out. This stage will take a bit of proper hammer usage!
Give the cups a good clean.
Give the thread in the frame a good clean.
7. Refit the cups to the frame. Don't forget the plastic central sleeve inbetween them and the spacers. Use copper grease on the threads. Remember which way each tightens from step 3?
8. Make sure the bearings are well packed with grease. I usually prise the seals off with the tip of a stanley knife then pack them with good quality Teflon grease.
9. Smear a coating of grease inside the cups where the new bearings will sit.
10. One at a time - press the new bearings into the cups. I use a headset press and one of the old bearings (see picture) to do this but you could easily and cheaply make your own press with some threaded bar etc from a DIY store.
11. Smear the bearing tops with grease and then re-insert the rubber seals and the plastic covers.
12. Push the drive side crank back through (don't forget to put the chain round it!). Grease the splines and then attach the non-drive side crank. Insert the plastic cap and tighten it up using the install tool. Nip up the two allen bolts (not too tight) alternating between the two.